1. Field of the Invention
This invention relates to equalizers in general and more particularly, to an improved equalizer for handling QPSK signals that have been subjected to various types of distortions.
2. Prior Art
Extensive usage of digital data communication systems have placed an increased emphasis on optimizing RF wide band data transmission utilizing modulation techniques. In this connection, serious efforts have been directed in the recent years to improve the modulation techniques for high speed data transmission. Generally, the modulation scheme used entails a trade-off between signal power and bandwidth, usually in favor of bandwidth in order to provide faster data transmission in a limited bandwidth. More recently, quadriphase shift keyed modulation (QPSK) have become popular as opposed to the usual biphase shift keyed modulation (BPSK).
QPSK modulation provides a twofold increase in the signal data transmission with identical bandwidths compared to BPSK modulation. In theory identical performance is possible with QPSK as compared to BPSK when twice the transmitted power is used. In practice, however, it is found that this theoretical level of performance is not attainable in QPSK systems because of distortions due to the additive noise, hardware deficiencies, and media dispersion on the received signal most of which are more pronounced in QPSK systems than in BPSK systems. In particular, it has been found that the phasor distortions resulting from such things as typical modulator and demodulator hardware and mistuned bandpass filters are often extremely severe. These distortions tend to compromise the otherwise advantageous characteristics of a higher data transmission speed of a QPSK modulation system and compound the adverse effect already imposed on the signal by the additive noise.
In handling the distortions of the QPSK signals, various equalization approaches have been utilized to date. Of these, the so-called linear adaptive equalizer, such as that described in "Simultaneous Adaptive Estimate and Decision Algorithm for Carrier Modulated Data Transmission System" by H. Kobayashi, IEEE Transaction on Communications Technology, Vol. COM 19, June, 1971 pages 268-280 appear to be popular. For the QPSK configuration, the linear adaptive equalizer usually includes the inphase (I) and quadriphase (Q) channel adaptive filters for providing the equalization to the incoming QPSK demodulated signals.
The adaptive equalizer filter is generally configured in the form of tapped delay line transversal filters with each tap being weighted and the taps being summed together. The tap weightings determine the filter characteristics. Generally, the circuit means utilized for deriving the two channel filters are designed to be identical by assuming that the inphase and quadrature signals have been subjected to distortions of identical nature and that, therefore, the same equalization filtering is required for the two incoming QPSK demodulated signals, i.e., the inphase and quadrature signals.